Such a suspension system is designed to form the connecting interface between a turbo-engine and a wing of the aircraft. It transmits forces generated by the associated turbo-engine to the aircraft structure, and also is used to route fuel, electrical and hydraulic and air systems between the engine and the aircraft.
The suspension system comprises a rigid structure also called the primary structure, often a <<box>> type structure, in other words formed by the assembly of lower and upper spars and side panels connected to each other by cross stiffening ribs, in order to transmit forces.
The system is also provided with suspension means inserted between the turbo-engine and the rigid structure, these means globally comprising two engine suspensions, and a system for resisting thrusts generated by the turbo-engine. In prior art, this force resistance system normally comprises two side connecting rods connected firstly to an aft part of the turbojet fan casing, and secondly to an aft suspension fixed onto the central part of the turbojet.
In the same way, the suspension system also comprises another series of suspensions forming a suspension system inserted between the rigid structure and the aircraft wing, this system normally being composed of two or three suspensions.
Moreover, the pylon is provided with a plurality of secondary structures segregating and maintaining the systems while supporting pylon fairing elements, which are usually in the form of assemblies of panels added onto the structures. As is well known to those skilled in the art, secondary structures are differentiated from the rigid structure by the fact that they are not designed to transfer forces from the engine to be transmitted to the aircraft wing.
The secondary structures include the aft pylon fairing, also called the APF, that has a plurality of functions including the formation of a thermal or fire resistant barrier, and the formation of aerodynamic continuity between the engine exhaust and the suspension pylon.
The aft pylon fairing is usually in the form of a box comprising two side panels assembled to each other by inner stiffening cross-ribs spaced at intervals from each other along a longitudinal direction of the fairing, and a heat protection deck. Note that this box is not necessarily closed opposite the heat protection deck, in other words in the upper part when the engine will be suspended under the aircraft wing, considering that this is the location at which it is connected to the other pylon structures.
The heat protection deck is provided with an outer surface designed to delimit the engine core flow which is delimited by this deck, while the engine fan flow is delimited by the outer surfaces of the side panels, due to their location in the annular fan flow duct of the engine, and/or at the engine exhaust.
In some solutions according to prior art, the opposite ends of the heat protection deck are mounted fixed to the corresponding two side panels that also match the cross ribs. Alternately or simultaneously, the deck is fixed onto the cross ribs.
In this configuration the heat protection deck is in contact with the very hot core engine flow, which means that it deforms strongly due to thermal expansion. However, its corresponding embedments into the lower ends of the two side panels and/or the lower ends of the cross-ribs create high thermomechanical stresses within the deck and the elements into which it is embedded, which is obviously not good for these elements.
Note that this phenomenon through which high thermomechanical stresses are introduced due to the large thermal expansion of the deck is accentuated by the fact that the side panels are immersed in the fairly cool fan flow, such that their deformation caused by thermal expansion is very small. Nevertheless, they are affected by significant deformation caused by the stresses set up resulting from the expansion of the deck to which they are directly and rigidly connected, which degrades their aerodynamic shape and more generally deteriorates the global aerodynamic quality of the fairing. Naturally, such degradation increases the generated parasite drag.
Consequently, note that the aerodynamic quality of the fairing is also degraded by local deformations of the heat protection deck which cannot expand freely unstressed, because it is built into some fairing elements such as the inner ribs, as described above. Since the core engine flow is a very fast jet, local deformations encountered at the deck create a fairly large parasite drag.
Finally, note that the fairly cool fan flow is not directly delimited by the surface of the inner cross ribs due to their location inside the box, and the ribs may be sensitive to heat transferred from the heat protection deck with which they are in contact. Thus, to enable these ribs to perform their function to mechanically support the different elements of the box-shaped fairing, it may be necessary to oversize them and/or to use expensive materials with good heat resistance properties to manufacture them.
To solve the problems that have just been mentioned, it is proposed to shift the heat protection deck in order to move it away from the side panels and the transverse ribs. This embodiment is known particularly from documents EP 2 190 739 and U.S. Ser. No. 12/677,139.
The deck is shifted from the transverse inner ribs by means of longitudinal walls, it being understood that these are the same walls that are indirectly used to mount the deck on the ribs. Thus, the deck is no longer mounted on the ribs directly, which advantageously allows the deck to deform more freely due to thermal expansion following the large amount of heat released by the core engine flow delimited by this deck.
Nevertheless, there is still a need to optimise the design of the fairing, such that the fairing enables greater freedom of expansion of the heat protection deck along the longitudinal direction.